1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a television set, a computer, a word processor, an OA (Office Automation) apparatus, or the like, and a method for driving such a liquid crystal display device.
2. Description of the Related Art
FIG. 20 illustrates an equivalent circuit diagram of a typical active matrix type licquid crystal display device.
The liquid crystal display device includes a plurality of gate electrodes 61 and a plurality of source electrodes 62 crossing the gate electrodes 61. A switching element 63, e.g., a thin film transistor, is provided in the vicinity of each intersection of the gate electrode 61 and the source electrode 62. The display area is divided into a plurality of pixel regions arranged in a matrix, which are partitioned from one another by the gate electrodes 61 and the source electrodes 62. A pixel electrode 64 is provided in each of the pixel regions. The pixel electrode 64 is connected to the source electrode 62 via the switching element 63. A liquid crystal layer (not shown) is interposed between the pixel electrode 64 and a common electrode 65. A liquid crystal capacitor 66 is provided between the pixel electrode 64 and the common electrode 65. An image is displayed by holding an intended voltage in each liquid crystal capacitor 66.
FIG. 21 illustrates typical voltage waveforms used for driving such an active matrix type liquid crystal display device.
A pulse waveform of ON voltage Vgh for turning ON the switching element 63 is applied as a gate voltage Vg to the gate electrodes 61 for each field period. A plurality of such pulses are sequentially applied to scan the entire frame. A voltage corresponding to an image signal for the row for which the ON voltage Vgh is being input to the gate electrode 61 is applied as a source voltage Vs to the source electrode 62. A plurality of such image signals are applied sequentially. As a common voltage Vc, xc2x1Vcac is applied to the common electrode 65. Each time the ON voltage Vgh is applied to the gate electrode 61, a new source voltage Vs is applied to the pixel electrode 64 as a pixel voltage Vp. As a result, a liquid crystal voltage Vlc, which is equal to the difference between the pixel voltage Vp and the common voltage Vc, is applied through the liquid crystal layer.
FIG. 22 illustrates the relationship between the liquid crystal voltage Vlc and the transmission. FIG. 22 shows an example for a TN (twisted nematic) type normally white mode which has been used in the art as a typical liquid crystal display mode.
The output range xc2x1Vsmax of the source voltage Vs is normally set to the voltage difference between a voltage Vlca at which the 100% transmission is obtained and a voltage Vlcc at which about 1% transmission is obtained. Thus, the output range of the source voltage Vs is set to a minimum range required to obtain a practically sufficient contrast. When the output range of the source voltage Vs is set to be higher than this, a source driver having a high voltage resistance is required, thereby increasing the cost of the device. Therefore, the source voltage Vs and the common voltage Vc with respect to the liquid crystal voltage Vlc are set as shown in the following Expression 1.
Vlca=┌+Vsmaxxc2x1Vcac|
Vlcb=└+Vcac|
Vlcc=└+Vsmaxxc2x1Vcac|xe2x80x83xe2x80x83Expression 1
FIG. 23 illustrates transmission response characteristics of the liquid crystal panel. In FIG. 23, a solid line shows the change in the transmission obtained when the image signal is changed from white displayxe2x86x92black displayxe2x86x92white display, and a broken line shows the change in the transmission obtained when the image signal is changed from white displayxe2x86x92gray-level displayxe2x86x92white display.
FIG. 23 shows that when the image signal is changed from white displayxe2x86x92black displayxe2x86x92white display, each transmission response is substantially completed within one field period. However, when the image signal is changed from white displayxe2x86x92gray-level displayxe2x86x92white display, the transmission does not change to a transmission corresponding to the image signal within one field period. Thus, between two image signals where the voltage difference is small, the transmission response may be slow.
Liquid crystal display devices have been wide spread as thin display devices as the image qualities thereof, e.g., the contrast, the brightness, and the color reproducibility, have been improved and are now comparable to those of other types of display devices such as CRTs. However, liquid crystal display devices have a relatively slow response as described above. Therefore, when displaying a motion picture on a liquid crystal display device, the displayed picture may be blurred or the after-image phenomenon may occur. This drawback has prevented liquid crystal display devices from replacing CRTs in some applications.
Japanese Laid-Open Publication No. 56-27198 discloses a display device which produces a color display by using a black and white liquid crystal panel in combination with a light source whose output color is switched among red, blue and green. This is called a xe2x80x9cfield sequential color methodxe2x80x9d.
In this method, the output color of the light source is switched among red, blue and green while an image corresponding to each output color is synchronously displayed. Therefore, when displaying a color image, the image signal changes for each display operation even if the image is stationary. Thus, when the response of the liquid crystal panel is slow, the color information for one display operation and the color information for the next display operation may be mixed together, thereby reducing the color reproducibility. In such a case, it is difficult to realize a sufficient display performance.
As described above, the slow response of the liquid crystal panel has been a drawback which deteriorates the display performance. In order to address such a drawback, various methods have been proposed in the art as follows.
For example, Japanese Laid-Open Publication No. 4-42211 proposes a method in which a voltage corresponding to an assist signal, which is different from an image signal, is applied before applying a voltage corresponding to the image signal. This method is based on the fact that the effective liquid crystal response speed (i.e., the speed of the response of the liquid crystal molecules to an applied voltage) can be increased by applying a voltage which is larger or lower than the voltage corresponding to the image signal through the liquid crystal layer before applying the voltage corresponding to the image signal therethrough. This method improves the motion picture display quality.
xe2x80x9cSID 98 DIGEST P. 143 A Novel Wide-Viewing Angle Motion-Picture LCDxe2x80x9d proposes another method in which a voltage corresponding to an assist signal is applied before applying a voltage corresponding to an image signal. This method improves the motion picture display quality by erasing the displayed image before displaying the next image.
Japanese Laid-Open Publication No. 9-138421 proposes a driving method in which a voltage corresponding to an assist signal is applied by activating all scanning lines and then providing the assist signal by varying the common voltage at each common electrode before applying the voltage corresponding to the image signal.
It is possible to prevent the color reproducibility from being reduced by applying the above-described improvements to the field sequential color method.
As described above, it is possible to increase the liquid crystal response by providing a period for the application through the liquid crystal panel of a voltage corresponding to an assist signal, which is not an image signal, before the period for the application of a voltage corresponding to the image signal. In order to effectively improve the liquid crystal response, it is important to appropriately set the voltage value of the assist signal and the length of the period for the application of the voltage corresponding to the assist signal.
Generally, as the amount of change in the voltage to be applied through the liquid crystal layer is larger, it is possible to impart a larger energy to the liquid crystal molecules thereby increasing the response of the display device. Therefore, the voltage value corresponding to the assist signal is preferably set to a voltage value which exceeds the voltage range for the image signals. It is further preferred that the voltage corresponding to the assist signal is variable according to the voltage corresponding to the image signal.
The above-described driving method increase the response speed of the liquid crystal panel by utilizing the transitional response in the period for the application of the voltage corresponding to the assist signal. Therefore, it is preferred that the period for the application of the voltage corresponding to the assist signal can be set to an optimal period for the response characteristics of the liquid crystal panel to be used.
However, the above-described driving method suffers from the following operational limitations, and it has been difficult to effectively improve the liquid crystal response speed.
In the method of Japanese Laid-Open Publication No. 4-42211, a voltage corresponding to an assist signal, which is different from an image signal, is applied from a source electrode through a pixel electrode via a switching element before applying a voltage corresponding to the image signal from the source electrode through the pixel electrode via the switching element. Then, in order to apply a voltage which exceeds the voltage range for the image signals through the liquid crystal layer, it is necessary to increase the voltage resistance of the source driver for inputting the signal voltage to the source electrode, thereby increasing the production cost. Moreover, since the voltage corresponding to the assist signal is applied via the switching element, it is necessary to apply the voltage corresponding to the assist signal within a short period which is provided in addition to the period for the application of the voltage corresponding to the image signal. To do so, it is necessary to dramatically improve the performance of the switching element. Therefore, it is difficult to employ the method in practical use.
In the method of xe2x80x9cSID 98 DIGEST P. 143 A Novel Wide-Viewing Angle Motion-Picture LCDxe2x80x9d, the scanning operation for applying a voltage corresponding to an image signal from a source electrode to a pixel electrode via a switching element and the scanning operation for applying a voltage corresponding to an assist signal from the source electrode to the pixel electrode via the switching element are repeated. Again, in order to apply a voltage which exceeds the voltage range for the image signals through the liquid crystal layer, it is necessary to increase the voltage resistance of the source driver for inputting the signal voltage to the source electrode, thereby increasing the production cost. Moreover, during the period in which the voltage corresponding to the assist signal is applied to the liquid crystal panel, an image signal cannot be displayed, thereby lowering the brightness of the display screen. In order to prevent such reduction of the brightness, the assist signal period is preferably set to be short. It is further preferred that the period in which the voltage corresponding to the assist signal is applied to the liquid crystal panel is set to a minimum period required for ensuring a sufficient liquid crystal response. However, in order to shorten the period in which the voltage corresponding to the assist signal is applied to the liquid crystal panel in this driving method, it is necessary to shorten the assist signal writing scanning period (i.e., the period for a scanning operation for writing the assist signal). To do so, it is necessary to increase the size of the switching element so as to improve the performance of the switching element. Thus, there may occur problems such as an increased defect rate for the switching element, thereby increasing the production cost.
In the method of Japanese Laid-Open Publication No. 9-138421, a voltage corresponding to an assist signal is applied after activating all scanning lines, whereby a constant voltage is applied through all pixels as the voltage corresponding to the assist signal. In an actual image display operation, however, the voltage corresponding to the image signal varies for various pixels, whereby it is necessary to set assist signals respectively corresponding to different image signals for the various pixels in order to reduce the response speed for each pixel, i.e., in order to reduce the amount of time required for the transmission of each pixel to reach the transmission corresponding to the image signal for that pixel. Therefore, the response speed cannot be improved effectively by using such an assist signal that is constant and does not correspond to the image signal for each pixel. Moreover, when writing the assist signal, all of the gate electrodes are activated, and the capacity load per source electrode increases, whereby it is necessary to provide a high-performance source driver capable of driving such an increased capacity load.
Moreover, in each of the above-described driving methods, the voltage corresponding to the assist signal is applied via the switching element, thereby increasing the amount of power consumed by the source driver when writing the assist signal, and thus deteriorating a feature of liquid crystal display devices, i.e., a small power consumption.
According to one aspect of this invention, a liquid crystal display device includes: a plurality of first electrodes; a plurality of second electrodes crossing the first electrodes; a plurality of switching elements each provided in a vicinity of an intersection of the first electrode and the second electrode; a plurality of pixel electrodes respectively provided in a plurality of regions which are arranged in a matrix and partitioned from one another by the first electrodes and the second electrodes; a plurality of first electrodes to each of which a gate voltage is applied for turning ON/OFF each of the switching elements; a plurality of second electrodes to each of which a source voltage is applied; and a plurality of third electrodes to each of which a common voltage is applied, the third electrodes being arranged so that a liquid crystal layer is interposed between the third electrodes and the pixel electrodes. The source voltage includes a voltage corresponding to an image signal and another voltage corresponding to an assist signal. The common voltage has different values between an image signal writing scanning period and at least one assist signal writing scanning period, the image signal writing scanning period being defined as a period during which the voltage corresponding to the image signal is applied, and the assist signal writing scanning period being defined as a period during which the voltage corresponding to the assist signal is applied.
In one embodiment of the invention, a liquid crystal capacitor is provided between one of the pixel electrodes and one of the third electrodes.
In one embodiment of the invention, a liquid crystal capacitor is provided between one of the pixel electrodes and one of the second electrodes.
According to another aspect of this invention, a liquid crystal display device includes: a plurality of first electrodes; a plurality of second electrodes crossing the first electrodes; a plurality of switching elements each provided in a vicinity of an intersection of the first electrode and the second electrode; a plurality of pixel electrodes respectively provided in a plurality of regions which are arranged in a matrix and partitioned from one another by the first electrodes and the second electrodes; a plurality of first electrodes to each of which a gate voltage is applied for turning ON/OFF each of the switching elements; a plurality of second electrodes to each of which a source voltage is applied; and a plurality of third electrodes to each of which a common voltage is applied, the third electrodes being arranged so that a liquid crystal layer is interposed between the third electrodes and the pixel electrodes. The liquid crystal display device further includes a plurality of assist electrodes to each of which an assist voltage is applied, the assist electrode having an assist capacitor being provided between each of the pixel electrodes and the assist electrode. An image signal application period is defined as a period during which a voltage corresponding to an image signal is applied between the pixel electrode and the third electrode. An assist signal application period is defined as a period during which a voltage corresponding to an assist signal is applied between the pixel electrode and the third electrode. At least one of the common voltage and the assist voltage has different values between the image signal application period and the assist signal application period.
According to still another aspect of this invention, a liquid crystal display device includes: a plurality of first electrodes; a plurality of second electrodes crossing the first electrodes; a plurality of switching elements each provided in a vicinity of an intersection of the first electrode and the second electrode; a plurality of pixel electrodes respectively provided in a plurality of regions which are arranged in a matrix and partitioned from one another by the first electrodes and the second electrodes; a plurality of first electrodes to each of which a gate voltage is applied for turning ON/OFF each of the switching elements; a plurality of second electrodes to each of which a source voltage is applied; and a plurality of third electrodes to each of which a common voltage is applied, the third electrodes being arranged so that a liquid crystal layer is interposed between the third electrodes and the pixel electrodes. The liquid crystal display device further includes a plurality of assist electrodes to each of which an assist voltage is applied, the assist electrode having an assist capacitor being provided between each of the pixel electrodes and each of the first electrodes. An image signal application period is defined as a period during which a voltage corresponding to an image signal is applied between the pixel electrode and the third electrode. An assist signal application period is defined as a period during which a voltage corresponding to an assist signal is applied between the pixel electrode and the third electrode. At least one of the common voltage and the assist voltage has different values between the image signal application period and the assist signal application period.
According to still another aspect of this invention, there is provided a method for driving a liquid crystal display device. The liquid crystal display device includes: a plurality of first electrodes; a plurality of second electrodes crossing the first electrodes; a plurality of switching elements each provided in a vicinity of an intersection of the first electrode and the second electrode; a plurality of pixel electrodes respectively provided in a plurality of regions which are arranged in a matrix and partitioned from one another by the first electrodes and the second electrodes; a plurality of first electrodes to each of which a gate voltage is applied for turning ON/OFF each of the switching elements; a plurality of second electrodes to each of which a source voltage is applied; and a plurality of third electrodes to each of which a common voltage is applied, the third electrodes being arranged so that a liquid crystal layer is interposed between the third electrodes and the pixel electrodes. The source voltage includes a voltage corresponding to an image signal and another voltage corresponding to an assist signal. The method includes the steps of: applying the common voltage during an image signal writing scanning period which is defined as a period during which the voltage corresponding to the image signal is applied; and applying the common voltage during at least one assist signal writing scanning period which is defined as a period during which the voltage corresponding to the assist signal is applied. The common voltage has different values between the image signal writing scanning period and the at least one assist signal writing scanning period.
In one embodiment of the invention, a liquid crystal capacitor is provided between one of the pixel electrodes and one of the third electrodes.
In one embodiment of the invention, a liquid crystal capacitor is provided between one of the pixel electrodes and one of the second electrodes.
In one embodiment of the invention, a range of a voltage applied to the liquid crystal capacitor during each assist signal writing scanning period is greater than a range of a voltage applied to the liquid crystal capacitor during the image signal writing scanning period.
In one embodiment of the invention, a range of a voltage applied to the liquid crystal capacitor during each assist signal writing scanning period is greater than a range of a voltage applied to the liquid crystal capacitor during the image signal writing scanning period.
In one embodiment of the invention, the source voltage applied during the assist signal writing scanning period includes a voltage for producing a black display or a white display.
In one embodiment of the invention, the source voltage applied during the assist signal writing scanning period includes a maximum voltage or a minimum voltage which can be output by a source voltage generation circuit for generating the source voltage.
In one embodiment of the invention, one field period includes at least two subfield periods including the image signal writing scanning period and the assist signal writing scanning period. A voltage corresponding to an image signal for a predetermined color component for each of the subfield periods is applied as the source voltage during the image signal writing scanning period.
In one embodiment of the invention, the one field period includes: a subfield period for displaying a red component; a subfield period for displaying a green component; and a subfield period for displaying a blue component.
According to still another aspect of this invention, there is provided a method for driving a liquid crystal display device. The liquid crystal display device includes: a plurality of first electrodes; a plurality of second electrodes crossing the first electrodes; a plurality of switching elements each provided in a vicinity of an intersection of the first electrode and the second electrode; a plurality of pixel electrodes respectively provided in a plurality of regions which are arranged in a matrix and partitioned from one another by the first electrodes and the second electrodes; a plurality of first electrodes to each of which a gate voltage is applied for turning ON/OFF each of the switching elements; a plurality of second electrodes to each of which a source voltage is applied; and a plurality of third electrodes to each of which a common voltage is applied, the third electrodes being arranged so that a liquid crystal layer is interposed between the third electrodes and the pixel electrodes. The liquid crystal display device further includes a plurality of assist electrodes to each of which an assist voltage is applied, the assist electrode having an assist capacitor being provided between each of the pixel electrodes and the assist electrode. The method includes the steps of: applying a voltage corresponding to an image signal is applied between the pixel electrode and the third electrode during an image signal application period; and applying a voltage corresponding to an assist signal is applied between the pixel electrode and the third electrode during an assist signal application period. At least one of the common voltage and the assist voltage has different values between the image signal application period and the assist signal application period.
In one embodiment of the invention, a plurality of rows of assist electrodes receive alternately different signals.
In one embodiment of the invention, a voltage which takes two or more levels is applied during the assist signal application period to one of at least one of the third electrodes and at least one of the assist electrodes.
In one embodiment of the invention, a write period is provided during which a voltage corresponding to an image signal is applied to the pixel electrodes via the switching elements. A first assist signal application period including the write period and a second assist signal application period not including the write period are provided. A polarity of a voltage between the pixel electrodes and the third electrodes is reversed with respect to a polarity of a voltage applied during the image signal application period between the write period and the second assist signal application period.
In one embodiment of the invention, the voltage for the assist signal application period is simultaneously applied to a plurality of pixels.
In one embodiment of the invention, the assist signal application period is coordinated with a timing at which the voltage corresponding to the image signal is applied to each of the pixel electrodes.
In one embodiment of the invention, a voltage exceeding a voltage range to be applied during the image signal application period is applied between the pixel electrodes and the third electrodes during the assist signal application period.
In one embodiment of the invention, one field period includes at least two subfield periods including the image signal application period and the assist signal application period. A voltage corresponding to an image signal for a predetermined color component for each of the subfield periods is applied between the pixel electrodes and the third electrodes during the image signal application period.
In one embodiment of the invention, the one field period includes: a subfield period for displaying a red component; a subfield period for displaying a green component; and a subfield period for displaying a blue component.
According to still another aspect of this invention, there is provided a method for driving a liquid crystal display device. The liquid crystal display device includes: a plurality of first electrodes; a plurality of second electrodes crossing the first electrodes; a plurality of switching elements each provided in a vicinity of an intersection of the first electrode and the second electrode; a plurality of pixel electrodes respectively provided in a plurality of regions which are arranged in a matrix and partitioned from one another by the first electrodes and the second electrodes; a plurality of first electrodes to each of which a gate voltage is applied for turning ON/OFF each of the switching elements; a plurality of second electrodes to each of which a source voltage is applied; and a plurality of third electrodes to each of which a common voltage is applied, the third electrodes being arranged so that a liquid crystal layer is interposed between the third electrodes and the pixel electrodes. The liquid crystal display device further includes a plurality of assist electrodes to each of which an assist voltage is applied, the assist electrode having an assist capacitor being provided between each of the pixel electrodes and each of the first electrodes. The method includes the steps of: applying a voltage corresponding to an image signal is applied between the pixel electrode and the third electrode during an image signal application period; and applying a voltage corresponding to an assist signal is applied between the pixel electrode and the third electrode during an assist signal application period. At least one of the common voltage and the assist voltage has different values between the image signal application period and the assist signal application period.
In one embodiment of the invention, a voltage which takes two or more levels is applied during the assist signal application period to one of at least one of the third electrodes and at least one of the first electrodes.
In one embodiment of the invention, a write period is provided during which a voltage corresponding to an image signal is applied to the pixel electrodes via the switching elements. A first assist signal application period including the write period and a second assist signal application period not including the write period are provided. A polarity of a voltage between the pixel electrodes and the third electrodes is reversed with respect to a polarity of a voltage applied during the image signal application period between the write period and the second assist signal application period.
In one embodiment of the invention, the voltage for the assist signal application period is simultaneously applied to a plurality of pixels.
In one embodiment of the invention, the assist signal application period is coordinated with a timing at which the voltage corresponding to the image signal is applied to each of the pixel electrodes.
In one embodiment of the invention, a voltage exceeding a voltage range to be applied during the image signal application period is applied between the pixel electrodes and the third electrodes during the assist signal application period.
In one embodiment of the invention, one field period includes at least two subfield periods including the image signal application period and the assist signal application period. A voltage corresponding to an image signal for a predetermined color component for each of the subfield periods is applied between the pixel electrodes and the third electrodes during the image signal application period.
In one embodiment of the invention, the one field period includes: a subfield period for displaying a red component; a subfield period for displaying a green component; and a subfield period for displaying a blue component.
Thus, the invention described herein makes possible the advantages of: (1) providing a liquid crystal display device in which it is possible to increase the liquid crystal response speed, prevent a motion picture from being blurred or having an after-image, improve the color reproducibility in the field sequential color method, and reduce the production cost and the power consumption; and (2) providing a method for driving such a liquid crystal display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.